1. Field of the Invention
The present invention relates to a printing technique of transferring an overcoat onto a printed image in a thermal printer.
2. Description of the Related Art
Recently, a so-called home laboratory is popular to create a photograph by printing an image of image data acquired by a digital camera or image data processed by a computer onto a dedicated printing medium using a printer in the home and the like. The home laboratory often uses a thermal printer excellent in tonal expression of printing colors.
The thermal head of the thermal printer is configured by forming heaters in line (this line direction will be defined as the main scanning direction or the longitudinal direction of the thermal head).
The tonality of one pixel (one dot) is achieved by controlling the amount of dye of an ink ribbon sublimated per pixel. The dye sublimation amount is controlled by controlling energy to be applied to the heaters.
While conveying a printing medium in the printing direction in accordance with print data corresponding to image data (printing medium conveyance direction will be defined as the sub scanning direction), the heaters are selectively energized to sublimate the dye of the ink ribbon, forming pixels.
The ink ribbon is formed from Y (Yellow), M (Magenta), and C (Cyan) dye layers which give printing colors to a printing medium, and an overcoat layer (OC) which protects a printed image.
The thermal head presses the ink ribbon and printing medium to contact each other, and forms printing colors by sublimating Y, M, and C pixels by sequential scanning on the printing medium by an area corresponding to the number of pixels in the main scanning direction of the heaters×the number of pixels in the sub scanning direction. Then, the thermal head transfers an OC onto the printed image to protect the printing colors produced by the sublimated Y, M, and C.
The OC to be transferred by the thermal printer can change in thickness and surface state by controlling energy to be applied to the heaters.
Japanese Patent Laid-Open No. 2001-012996 discloses a printer apparatus which arbitrarily selects the presence/absence of gloss of the OC. The heating temperature to the OC is controlled while controlling energization to the thermal head to keep constant the amount of heat to be supplied to the OC. Changing the heating temperature to the OC changes the surface state of the OC and the degree of diffuse reflection on the OC surface. Based on this, the presence/absence of gloss of a printed material can be arbitrarily selected.
Japanese Patent Laid-Open No. 2002-240402 discloses a method of forming supplementary information about image information as a watermark character in the OC. Image information is received from a printing medium file or the like, and supplementary information about the image information is acquired. After printing the image information on a printing medium, application energy to the transfer head is controlled to change the glossiness of a film sheet. As a result, the supplementary information about the image information can be formed from a watermark character, sign, or the like on the sheet.
However, the printer apparatus in Japanese Patent Laid-Open No. 2001-012996 controls only the presence/absence of gloss of a printed material, and does not consider hiding of information on a printed material.
The method in Japanese Patent Laid-Open No. 2002-240402 forms supplementary information about image information as a watermark character on the surface of a printing medium without degrading the quality of a printed image. This method forms supplementary information about image information as the difference in glossiness on the surface of a printing medium so that the information can be visually checked. This method does not hide the information.